Whining Transmission in specific gears

Picked up a used '02 Taco from a dealership recently.
67K miles
4x4 standard cab w/ short bed

There's a whining noise when on the highway when in 5th gear and the rpm's are at about 2,400+
The whine sounds like a high pitched bearing noise coming from what seems to be the clutch/tranny area under the truck. (but that's just how I perceive it from within the cab - you know how reliable that can be).
In 5th gear it's loud enough to be unacceptable for a vehicle I just recently purchased. I didn't notice it while test driving. It's loud enough to have to turn the stereo up to make it go away.
I tried the same rpm's in 4th and 3rd and found it to be occurring in 3rd gear as well. Not as drastically, but it's present.
The truck has only been driven in 4High mode since I've owned it.

1. What are the possible causes?
2. The truck isn't "Pre-Owned Certified" but is under a Used Car Warranty required by the state of MA. The warranty defines the responsabiltiy of the dealership to be for Safety and Functional "Defects". Would I have a case against the dealership if I asked them to fix the issue even though its just a noise?

Thanks.

J

 

Manual transmission? Mine whines in fifth. I'm convinced it's a bearing in the transmission.